Abstract：At present，during the construction of advancing culvert box at home and abroad，the soil in front of box should be reinforced to retain stability. The tunnel of Shanghai Middle Ring Line located in the intersection of Hongxu Road and Beihong Road will be the first case to adopt pipe roof culvert box advancing scheme without reinforcing the saturated soft soil layer to decrease project cost. In order to sustain the stability of surface and control the ground settlement during the construction，the reasonable design of girding will be crucial. Based on the principle of mechanics and some hypothesis，this paper takes advantage of the balance between friction and earth pressure in grid，and presents a kind of self-balance design theory. A differential equation is set up and the distribution and attenuation rules of soil pressure，normal stress，shear stress and etc. are obtained. These rules of stress distribution and attenuation conform to a series of exponential equations. In addition，the critical length of grid and the relations among length，width and height have been obtained from the equations. Also，the main factors affecting the critical length of grid are analyzed from these equations and are obtained as follows：the soil pressure in front of grid；frictional property between soil and grid wall；soil friction angle and cohesion. Analysis indicates that the critical length is increased as the soil pressure increasing and decreased as the friction factor increasing. These equations and results can be regarded as academic foundation for grid design for soft soil layers.
